The invention relates to an apparatus for stretching the links, which are intercoupled by rocker pins, of an endless transmission chain, while surpassing the limit of elasticity thereof, said apparatus comprising a first pair and a second pair of conical supporting surfaces for the ends of the rocker pins, resting thereon, while protruding outside the links of the transmission chain which is arranged there around, said surfaces being carried by rotatingly supported shafts with a controlled mutual distance.